Feb 19
th
, 2009
1.
Force and energy
•
Newton’s laws tell us that unless we exert a force a body wont chance its motion
•
This implies that we need to do work to change motion, and leads to concepr of
energy
•
POTENTIAL ENERGY is sorted as a result of doing work against a force, e.g.,
lifting something up high
•
KINETIC ENERGY is a result of motion, since we know that work is required to
stop the motion
•
RADIAIVE energy s sorted in, or composed of light e.g., solar power
2.
Energy
a.
Is we care about gravity we can easily define potential kinetic energy
b.
PE=GMm/R
c.
KE=[mv(^2)]/2
d.
Note that total energy is conserved. This is a general rule
3.
escape velocity
a.
using these expression, we can determined the speed an object needs to
have in order to escape the gravitational field of the earth
b.
any less and the object will eventually fall back to earth
c.
another way of stating this is that KE must be greater then or equal to PE.
The “escape velocity” occurs when PE=KE
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 Spring '09
 Eisner
 Energy, Kinetic Energy, Light

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